Field
The disclosed technology generally relates to semiconductor devices, and more particularly to a non-volatile ferroelectric memory device and to methods of fabricating the same.
Description of the Related Technology
Memories and in particular non-volatile memories are in great demand, for instance for being used in high-performance digital cameras, mp3 players, flash drives and cards, mobile phones, personal digital assistants (PDAs) and ultra-portable notebook personal computers (PCs) where high-density, ultra-compact, and low power-consumption storage devices are needed to replace the use of bulky hard disk drives. With new applications coming up, such as distributed sensor nodes and the concept of the internet of things, future ICs will need local memory that is very low-power operable, as well as switching at low currents.
An example of a well-known non-volatile memory is flash memory; however, a main disadvantage thereof is that flash memory devices need high voltages (20V) for writing and erasing processes, which are incompatible with the CMOS baseline.
Another non-volatile memory which has attracted great attention from the research community as a good candidate for both memory and switching applications is the ferroelectric field-effect transistor (FeFET) memory device. A state-of-the-art FeFET memory device resembles very much a metal-oxide-semiconductor FET (MOSFET); however the gate oxide dielectric is replaced by a ferroelectric material for a FeFET memory device. By modulating the gate electrode accumulation or depletion at the ferroelectric-semiconductor (channel) interface will occur and thereby switch the FeFET on or off.
A one transistor (1T) ferroelectric memory cell has been proposed and experimentally studied in order to reduce the size of 1T-1C design with consequent advantages in terms of size, read-out operation and costs. However, one main drawback of FeFETs known in the art is their non-perfect non-volatile behavior: when the ferroelectric material, which serves as the gate dielectric, is programmed, the gate dielectric is polarized in such a way that the threshold voltage is shifted, similar to a flash device. However, since now the charge causing this effect is polarization charge, caused by an atom moving within the unit cell of the ferroelectric crystal, there is no direct leakage current causing the cell to discharge. There is, however, another effect causing the information to leak away, which is referred to as the depolarization field. In most cases the electric field over the materials in contact with the ferroelectric material can be different from zero even in the retention condition when no voltage is applied. As a consequence, there will also be an electric field over the ferroelectric material which is always opposite to the polarization (Gauss' law). That induced electric field will disadvantageously work against the polarization and hence depolarize the cell causing it to lose its content, thus resulting in poor non-volatile properties.
Therefor there is a need for novel and improved ferroelectric memory cells.